This invention pertains to microlithography, which involves the transfer of a pattern, usually defined by a reticle or mask, onto a xe2x80x9csensitivexe2x80x9d substrate using an energy beam. Microlithography is a key technique used in the manufacture of microelectronic devices such as integrated circuits, displays, thin-film magnetic heads, and micromachines. More specifically, the invention pertains to methods and devices, used in the context of a microlithography method and apparatus, respectively, for compensating for lateral shift accompanying a leveling tilt imparted to a leveling table, such as a wafer table, associated with the wafer stage.
As the density and miniaturization of microelectronic devices has continued to increase, the accuracy and resolution demands imposed on microlithographic methods and apparatus also have increased. Currently, most microlithography is performed using, as an energy beam, a light beam (typically deep UV light) produced by a high-pressure mercury lamp or excimer laser, for example. Emerging microlithographic technologies include charged-particle-beam (xe2x80x9cCPBxe2x80x9d; e.g., electron-beam) microlithography and xe2x80x9csoft-X-rayxe2x80x9d (or xe2x80x9cextreme UVxe2x80x9d) microlithography.
All microlithographic technologies involve pattern transfer to a suitable substrate, which can be, for example, a semiconductor wafer (e.g., silicon wafer), glass plate, or the like. So as to be imprintable with the pattern, the substrate typically is coated with a xe2x80x9cresistxe2x80x9d that is sensitive to exposure, in an image-forming way, by the energy beam in a manner analogous to a photographic exposure. Hence, a substrate prepared for microlithographic exposure is termed a xe2x80x9csensitivexe2x80x9d substrate.
Microlithography conventionally is performed using any of various basic approaches including xe2x80x9cdirect writing,xe2x80x9d xe2x80x9ccontact printing,xe2x80x9d and xe2x80x9cprojectionxe2x80x9d microlithgraphy. Projection microlithography is the most common.
Basic aspects of a modern microlithography apparatus (xe2x80x9cexposure apparatusxe2x80x9d) 10 are shown in FIG. 18, in the context of a projection-exposure apparatus. A pattern is defined on a reticle (sometimes termed a xe2x80x9cmaskxe2x80x9d) 12 mounted on a reticle stage 14. The reticle 12 is xe2x80x9cilluminatedxe2x80x9d by an energy beam (e.g., UV light, charged particle beam, X-rays) produced by a source 16 and passed through an illumination-optical system 18. As the energy beam passes through the reticle 12, the beam acquires an ability to form an image, of the illuminated portion of the reticle 12, downstream of the reticle 12. The beam passes through a projection-optical system 20 that focuses the beam on a sensitive surface of a substrate 22 held on a substrate stage (xe2x80x9cwafer stagexe2x80x9d or xe2x80x9cwafer XY stagexe2x80x9d) 24. As shown in the figure, the source 16, illumination-optical system 18, reticle stage 14, projection-optical system 20, and wafer stage 24 generally are situated relative to each other along an optical axis AX. The reticle stage 14 is movable at least in the X- and xcex83-directions via a stage actuator 26 (e.g., linear motor), and the positions of the reticle stage 14 in the X- and Y-directions are detected by respective interferometers 28. The apparatus 10 is controlled by a controller (computer) 30.
The substrate 22 (also termed a xe2x80x9cwaferxe2x80x9d) is mounted on the wafer stage 24 via a wafer chuck 32 and wafer table 34 (also termed a xe2x80x9cleveling tablexe2x80x9d). The wafer stage 24 not only holds the wafer 22 for exposure (with the resist facing in the upstream direction) but also provides for controlled movements of the wafer 22 in the X- and Y-directions as required for exposure and for alignment purposes. The wafer stage 24 is movable by a suitable wafer-stage actuator 23 (e.g., linear motor), and positions of the wafer stage 24 in the X- and Y-directions are determined by respective interferometers 25. The wafer table 34 is used to perform fine positional adjustments of the wafer chuck 32 (holding the wafer 22), relative to the wafer stage 24, in the X-, Y-, and Z-directions. Positions of the wafer table 34 in the X- and Y-directions are determined by respective wafer-stage interferometers 36.
The wafer chuck 32 is configured to hold the wafer 22 firmly for exposure and to facilitate presentation of a planar sensitive surface of the wafer 22 for exposure. The wafer 22 usually is held to the surface of the wafer chuck 32 by vacuum, although other techniques such as electrostatic attraction can be employed under certain conditions. The wafer chuck 32 also facilitates the conduction of heat away from the wafer 22 that otherwise would accumulate in the wafer during exposure.
Movements of the wafer table 34 in the Z-direction (optical-axis direction) and tilts of the wafer table 34 relative to the Z-axis (optical axis AX) typically are made in order to establish or restore proper focus of the image, formed by the projection-optical system 20, on the sensitive surface of the wafer 22. xe2x80x9cFocusxe2x80x9d relates to the position of the exposed portion of the wafer 22 relative to the projection-optical system 20. Focus usually is determined automatically, using an auto-focus (AF) device 38. The AF device 38 produces data that is routed to the controller 30. If the focus data produced by the AF device 38 indicates existence of an out-of-focus condition, then the controller 30 produces a xe2x80x9cleveling commandxe2x80x9d that is routed to a wafer-table controller 40 connected to individual wafer-table actuators 40a. Energization of the wafer-table actuators 40a results in movement and/or tilting of the wafer table 34 serving to restore proper focus.
Details of a conventional scheme for tilting of the wafer table 34 are shown in FIG. 19, which shows the wafer stage 24, wafer-stage actuator 23, wafer table 34, and interferometer 36. Relative to the wafer stage 24, the wafer table 34 is supported by the wafer-table actuators 40a. Normally, three wafer-table actuators 40a are provided, supporting the wafer table 34 in a tripod manner, relative to the wafer stage 24, at respective xe2x80x9cpush points.xe2x80x9d The wafer-table actuators 40a can be, for example, piezo-electric actuators.
FIG. 19 depicts two closed-loop control systems. A first control system 42 pertains to tilting of the wafer table 34 relative to the wafer stage 24. A second control system 44 pertains to lateral (X-Y) positioning of the wafer stage 34. The first control system 42 is diagrammed as including a comparator 45, a controller 46, and a converter 47. A leveling command from the controller 30 responsive to an AF condition detected by the AF device 38 (FIG. 18) is routed to the comparator 45. The comparator 45 also is connected to a feedback loop 48 discussed below. The output signal of the comparator 45 is routed to the controller 46, which processes the signal according to a respective transfer function GWT. The processed signal from the controller 46 is routed to the converter 47, which converts the processed signal to a torque command (voltage) applied to the wafer-table actuators 40a. The resulting energization of the actuators 40a causes the wafer table 34 to tilt relative to the wafer stage 24 and relative to a line LAX parallel to the optical axis AX. The resulting angular rotation of the wafer table 34 is denoted by xcex8. Data concerning xcex8 is fed back from leveling-table-tilt sensors (not shown) via the feedback loop 48 to the comparator 45.
The second control system 44 includes a comparator 50, a wafer-stage controller 51, and a converter amplifier 52. A stage-position command from the controller 30 is routed to the comparator 50. The comparator 50 is connected to a feedback loop 53 discussed below. The output signal from the comparator 50 is routed to the controller 51, which processes the signal according to a respective transfer function GWS. The processed signal from the controller 51 is routed to the converter amplifier 52, which converts the processed signal to a voltage applied to the wafer-stage actuator 23. The applied voltage causes the wafer-stage actuator 23 to move a corresponding distance. The feedback loop 53 routes an output signal from the wafer-table interferometer 36 to the comparator 50.
As shown in FIG. 19, the center of rotation 54 of the wafer table 34 is not at the height of the wafer table 34 but rather is situated a distance L below the wafer table 34. If the wafer table 34 tilts the angle xcex8 in response to the leveling command, the edge of the wafer table 34 effectively experiences a lateral shift of xcex94x=L sin xcex8. This lateral shift is detected by the wafer-table interferometer 36, and the corresponding data is fed back to the comparator 50. As a result of this feedback, the wafer-stage actuator 23 can make a compensating movement of the wafer stage 24.
Many lithographic exposures are made in a scanning manner, wherein the reticle stage 14 and wafer stage 24 undergo synchronous motion relative to each other as the pattern is being exposed onto the wafer. Conventional wafer stages are massive (hundred kilograms or more) and consequently have a relatively slow response time to a stage-position command. As a result, a wafer stage 24 provided with a feedback loop 53 as shown in FIG. 19 simply is incapable of making a sufficiently rapid lateral movement to compensate for a change in tilt of the wafer table 34, especially for scanning exposures. As a result, significant inaccuracy is introduced into the exposure.
In view of the shortcomings of conventional apparatus as summarized above, an object of the invention is to provide apparatus and methods providing improved and more accurate compensations for lateral displacement of the leveling table (wafer table) arising during tilting motions of the leveling table.
As used herein, xe2x80x9ccompensationxe2x80x9d is not limited in meaning to a complete offset of the lateral displacement of the leveling table. Desirably, the amount of compensation is at least effective in reducing the lateral displacement to within alignment specifications. However, any reduction of lateral displacement, compared to not reducing the lateral displacement at all, falls within the scope of xe2x80x9ccompensation.xe2x80x9d
xe2x80x9cTiltxe2x80x9d and xe2x80x9ctiltingxe2x80x9d of the leveling table is any change in angle (xcex8) of the plane of the leveling table, relative to a line (Z-direction line parallel to the optical axis), from a previous angle at which the leveling table was positioned.
A xe2x80x9cposition-loop servoxe2x80x9d is a closed-loop feedback-control system governing position of a body such as the wafer stage, leveling table (wafer table), and/or reticle stage. Achieving an actual position (in the X-, Y-, and/or Z-axis direction, or a xcex8-direction) is performed by a suitable actuator such as a linear motor (wafer stage and reticle stage) or tilting mechanism (leveling table).
To achieve ends as summarized above, a first aspect of the invention is directed, in the context of exposure apparatus and methods, to control systems for compensating for lateral shift of the leveling table caused by a tilt (xcex8) of the leveling table. The contextual exposure apparatus includes a wafer stage that is movable at least in mutually perpendicular X- and Y-directions and the leveling table that is tiltable relative to a Z-axis (that is perpendicular to the X-and Y-directions). An embodiment of the control system comprises a wafer-stage-position loop servo, a leveling-table tilt-position loop servo, and a feed-forward loop from the leveling-table tilt-position loop servo to the wafer-stage-position loop servo. The wafer-stage-position loop servo is configured to actuate movement of the wafer stage in response to a positional command. The leveling-table tilt-position loop servo is configured to apply a tilting torque to the leveling table in response to a leveling command. The feed-forward loop is configured to convert a torque-control signal for the leveling table to a linear-acceleration-control signal for the wafer stage. The linear-acceleration-control signal causes the wafer stage to move laterally in a manner that compensates for the lateral shift of the leveling table accompanying a change in tilt of the leveling table.
In the embodiment summarized above, the feed-forward loop can be represented by a block diagram that includes a coordinate converter, a controller, and an adder. The coordinate converter is connected to the leveling-table tilt-position loop servo and is configured to convert an angular acceleration (xcex8xe2x80x3, or second derivative of xcex8) command from the leveling-table tilt-position loop servo to a corresponding linear-acceleration command output from the coordinate converter. The controller is connected downstream of the coordinate converter and is configured to apply a factor ms to the linear-acceleration command output from the coordinate converter (wherein ms is a combined mass of the wafer stage and leveling table), thereby producing a first force command. The adder is situated and configured to add the first force command to a second force command produced by the wafer-stage-position loop servo, so as to produce an output routed to a wafer-stage actuator.
The exposure apparatus can further include a reticle stage movable at least in the X- and Y-directions. The reticle stage is controlled by a reticle-stage-position loop servo configured to actuate movement of the reticle stage in response to a reticle-position command. In this configuration, the control system can further comprise a second feed-forward loop from the leveling-table tilt-position loop servo to the reticle-stage-position loop servo. The second feed-forward loop is configured to convert a positional signal for the leveling table to a position-control signal for the reticle stage. The position-control signal causes the reticle stage to move laterally to compensate, at least in part, for the lateral shift of the leveling table accompanying a change in tilt of the leveling table.
In another embodiment, the exposure apparatus includes a reticle stage that is movable at least in the X- and Y-directions, as summarized above. The control system comprises the reticle-stage-position loop servo and leveling-table tilt-position loop servo as summarized above. The control system also includes a first feed-forward loop from the leveling-table tilt-position loop servo to the reticle-stage-position loop servo. The reticle-stage-position loop servo is configured to actuate movement of the reticle stage in response to a reticle-position command. The leveling-table tilt-position loop servo is configured as summarized above. The first feed-forward loop is configured to convert a torque-control signal for the leveling table to a linear-acceleration-control signal for the reticle stage. The linear-acceleration-control signal causes the reticle stage to move laterally to compensate for the lateral shift of the leveling table accompanying a change in tilt of the leveling table. The first feed-forward loop can be represented by a block diagram that includes a controller and an adder. The controller is connected to the leveling-table tilt-position loop servo and is configured to convert a torque command from the leveling-table tilt-position loop servo to a corresponding linear-acceleration or force command output from the controller. The adder is situated and configured to add the command output from the controller to a force command produced by the reticle-stage-position loop servo, so as to produce an output routed to a reticle-stage actuator to cause compensatory motion of the reticle stage.
The control system summarized in the previous paragraph also can include a wafer-stage-position loop servo and a second feed-forward loop. The wafer-stage-position loop servo is configured, as summarized above, to actuate movement of the wafer stage in response to a wafer-position command. The second feed-forward loop extends from the leveling-table tilt-position loop servo to the wafer-stage-position loop servo. The second feed-forward loop is configured to convert a torque-control signal for the leveling table to a linear-acceleration-control signal for the wafer stage. The linear-acceleration-control signal causes the wafer stage to move laterally in a manner that compensates, at least in part, for the lateral shift of the leveling table accompanying a change in tilt of the leveling table. Any remaining compensation can be made by lateral motion of the reticle stage as controlled by the first feed-forward loop.
The control system summarized in the previous paragraph also can include a leveling-table tilt sensor located within a feedback loop of the leveling-table tilt-position loop servo. With such a configuration, the second feed-forward loop can be represented by a block diagram that includes a second controller and a second adder. The second controller is situated and configured to receive a xcex8 output signal from the leveling-table tilt sensor and to convert the xcex8 output signal to a corresponding positional signal routed to the second adder. The second adder is configured to add the xcex8 output signal to a wafer-stage-position command routed to the wafer-stage-position loop servo.
According to another aspect of the invention, exposure apparatus are provided. An embodiment of such an apparatus comprises a projection-optical system, a wafer stage, a leveling table, a wafer-stage-position loop servo, a leveling-table tilt-position loop servo, and a control system. The projection-optical system has an optical axis parallel to the Z-axis. The wafer stage is situated downstream of the projection-optical system and is movable at least in the X- and Y-axis directions. The leveling table is situated downstream of the projection-optical system and is tiltable relative to the Z-axis. The wafer-stage-position loop servo is connected to the wafer stage and is configured to actuate movement of the wafer stage in response to a wafer-position command. The leveling-table tilt-position loop servo is connected to the leveling table and is configured to apply a tilting torque to the leveling table in response to a leveling command. The control system compensates for lateral shift of the leveling table caused by a tilt (xcex8) of the leveling table. The control system comprises a feed-forward loop from the leveling-table tilt-position loop servo to the wafer-stage-position loop servo. The feed-forward loop is configured to convert a torque-control signal for the leveling table to a linear-acceleration-control signal for the wafer-stage. The linear-acceleration-control signal causes the wafer stage to move laterally in a manner that compensates for the lateral shift of the leveling table accompanying a change in tilt of the leveling table. The feed-forward loop can be represented by a block diagram that includes a coordinate converter, a controller, and an adder, as summarized above.
The exposure apparatus also can include a reticle stage that is situated upstream of the projection-optical system and that is movable at least in the X- and Y-directions. A reticle-stage-position loop servo is connected to the reticle stage and is configured to actuate movement of the reticle stage in response to a reticle-position command. With such a configuration of an exposure apparatus, the control system desirably includes a second feed-forward loop from the leveling-table tilt-position loop servo to the reticle-stage-position loop servo. The second feed-forward loop is configured to convert a positional signal for the leveling table to a position-control signal for the reticle stage. The position-control signal causes the reticle stage to move laterally to compensate, at least in part, for the lateral shift of the leveling table accompanying a change in tilt of the leveling table.
Another embodiment of an exposure apparatus according to the invention comprises a projection-optical system, leveling table, reticle stage, leveling-table tilt-position loop servo, and reticle-stage-position loop servo all as summarized above. The apparatus also includes a control system for compensating for lateral shift of the leveling table caused by a tilt (xcex8) of the leveling table. The control system includes a first feed-forward loop from the leveling-table tilt-position loop servo to the reticle-stage-position loop servo. The first feed-forward loop is configured to convert a torque-control signal for the leveling table to a linear-acceleration-control signal for the reticle stage. The linear-acceleration-control signal causes the reticle stage to move laterally to compensate for the lateral shift of the leveling table accompanying a change in tilt of the leveling table.
The apparatus summarized in the preceding paragraph can include a reticle-stage actuator that is situated and configured to move the reticle stage in response to the reticle-position command. The reticle-stage actuator is connected to the first feed-forward loop and configured to move the reticle stage in response to the linear-acceleration-control signal to compensate for lateral shift caused by leveling.
The first feed-forward loop can be represented by a block diagram that includes a controller and an adder. The controller is connected to the leveling-table tilt-position loop servo and is configured to convert a torque command from the leveling-table tilt-position loop servo to a corresponding linear-acceleration command or force command output from the controller. The adder is situated and configured to add the command output from the controller to a force command produced by the reticle-stage-position loop servo, so as to produce an output routed to a reticle-stage actuator.
The apparatus can include a wafer stage and wafer-stage-position loop servo, as summarized above. In such a configuration, the control system can further comprise a second feed-forward loop from the leveling-table tilt-position loop servo to the wafer-stage-position loop servo. The second feed-forward loop is configured to convert a torque-control signal to the leveling table to a linear-acceleration-control signal for the wafer stage. The linear-acceleration-control signal causes the wafer stage to move laterally in a manner that compensates, at least in part, for the lateral shift of the leveling table accompanying a change in tilt of the leveling table. Any remaining compensation can be made by lateral motion of the reticle stage as controlled by the first feed-forward loop.
The apparatus can include a leveling-table-tilt sensor located within a feedback loop of the leveling-table tilt-position loop servo. In such a configuration, the second feed-forward loop can be represented by a block diagram that includes a second controller and a second adder. The second controller is situated and configured to receive a xcex8 output signal from the leveling-table tilt sensor and to convert the xcex8 output signal to a corresponding positional signal routed to the second adder. The second adder is configured to add the xcex8 output signal to a wafer-stage-position command routed to the wafer-stage-position loop servo.
Yet another aspect of the invention pertains to methods (in the context of an exposure method in which a substrate, mounted on a wafer stage, is exposed to a pattern defined by a reticle) for maintaining an alignment of the substrate for exposure. In an embodiment of such a method, the substrate is mounted on a leveling table that is tiltable relative to a Z-axis perpendicular to the X- and Y-axis directions. The leveling table is mounted on the wafer stage and is controlled by a leveling-table tilt-position loop servo that applies a tilting torque to the leveling table as required in response to a leveling command corresponding to a torque-control signal. The wafer stage is controlled by a wafer-stage-position loop servo that actuates movement of the wafer stage at least in the X- and Y-axis directions as required in response to a wafer-position command. In association with tilting of the leveling table in response to the tilting command, a torque-control signal is fed forward from the leveling-table tilt-position loop servo to the wafer-stage-position loop servo such that the torque-control signal is converted to an acceleration-control signal for the wafer stage. The acceleration-control signal causes the wafer stage to move laterally to compensate for a lateral shift of the leveling table caused by tilting of the leveling table.
The method can include mounting the reticle in a reticle stage movable at least in the X- and Y-axis directions and controlled by a reticle-stage-position loop servo that actuates movement of the reticle stage in response to a reticle-position command. In such an embodiment, three actions occur in association with tilting of the leveling table in response to the tilting command: (1) feeding forward the torque-control signal from the leveling-table tilt-position loop servo to the reticle-stage-position loop servo; (2) converting the fed-forward torque-control signal to a linear-acceleration-control signal for the reticle stage; and (3) applying the linear-acceleration-control signal to the reticle stage to cause the reticle stage to move laterally to compensate for a lateral shift of the leveling table accompanying a change in tilt of the leveling table.
In another embodiment of a method according to the invention, the substrate is mounted on a leveling table as summarized above. The leveling table is mounted on the wafer stage and is controlled by a leveling-table tilt-position loop servo that applies a tilting torque to the leveling table as required in response to a leveling command corresponding to a torque-control signal. The reticle is mounted on a reticle stage that is movable at least in the X- and Y-axis directions and is controlled by a reticle-stage-position loop servo that actuates movement of the reticle stage in response to a reticle-position command. In association with tilting of the leveling table in response to the tilting command, three actions occur: (1) feeding forward the torque-control signal from the leveling-table tilt-position loop servo toward the reticle-stage-position loop servo; (2) converting the fed-forward torque-control signal to an acceleration-control signal for the reticle stage; and (3) applying the acceleration-control signal to the reticle stage to cause the reticle stage to move laterally to compensate for a lateral shift of the leveling table accompanying a change in tilt of the leveling table.
The method summarized in the preceding paragraph can include the step, while causing the reticle stage to move laterally to compensate for the lateral shift of the leveling table, of inhibiting compensating motions of the wafer stage.
According to another aspect of the invention, positioning apparatus are provided for positioning a substrate. An embodiment of such an apparatus includes a first stage movable at least in a first direction and a second stage mounted on the first stage. The second stage is configured to retain a substrate and is tiltable relative to the first stage. The apparatus includes a control system connected to the first stage and the second stage. The control system includes a first-stage-position loop for the first stage, a second-stage-position loop for the second stage, and a feed-forward loop connected to the first-stage-position loop and the second-stage-position loop. The first-stage-position loop actuates movement of the first stage by utilizing a first-stage control signal. The second-stage-position loop actuates a tilting motion of the second stage by utilizing a second-stage control signal. The feed-forward loop converts the second-stage control signal to the first-stage control signal. The first-stage control signal causes the first stage to move in a manner that compensates for the lateral shift of the substrate accompanying a change in tilt of the second stage.
Another embodiment of a positioning apparatus includes a first stage and second stage as summarized above. The apparatus also includes a third stage that is movable at least in the first direction and configured for motion in a synchronous manner with the substrate moved by the first stage. A control system is connected to the first, second, and third stages. The control system includes a second-stage-position loop for the second stage and a third-stage-position loop for the third stage. A first feed-forward loop is connected to the second-stage-position loop and the third-stage-position loop. The second-stage-position loop actuates a tilting motion of the second stage by utilizing a second-stage control signal. The third-stage-position loop actuates movement of the third stage by utilizing a third-stage control signal. The first feed-forward loop converts the second-stage control signal to the third-stage control signal, and the third-stage control signal causes the third stage to move in a manner that compensates for alignment errors between the third stage and the substrate accompanying a change in tilt of the second stage.
In the embodiment summarized above, the control system can further include a first-stage-position loop for the first stage and a second feed-forward loop connected to the second-stage-position loop and the first-stage-position loop. The first-stage-position loop actuates movement of the first stage by utilizing a first-stage control signal, and the second feed-forward loop converts the second-stage control signal to the first-stage control signal. The first-stage control signal causes the first stage to move in a manner that compensates, at least in part, for alignment errors between the third stage and the substrate accompanying a change in tilt of the second stage. Remaining compensation is contributed by the third stage as controlled by the first feed-forward loop.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.